1. Field of the Invention
The present invention relates to a lead frame for semiconductor devices which does not contain lead, one of the environmentally harmful substances, to a process for producing the same and to a semiconductor device formed by using the above-mentioned lead frame.
2. Description of the Related Art
Among the materials used in lead frames for semiconductor devices, lead used in solder is known to be a substance particularly harmful to environment. When allowed to stand, lead dissolves out of solder and exerts adverse effects on human and other organisms. Accordingly, development of solder, soldering paste and the like which do not use lead has been forwarded in the electronic industry.
A lead frame plated with palladium on the whole surface as a substitute for lead-containing solder has been put to practical use. However, when palladium used as a simple element is subjected to heat at the die bonding process and the wire bonding step, the wetability of solder tends to deteriorate, causing a problem in the reliability of soldering at the time of surface mounting. In recent years, therefore, a lead frame has been proposed wherein a thin plating of gold has been applied to the surface of palladium as a protective film. However, when flash plating of gold is applied to the outermost surface of palladium, the adhesion of mold resin with the lead frame tends to be poor, so that it is necessary to use a mold resin which is improved in adhesion with gold but is of high cost. Moreover, palladium is limited as to countries that can supply it and frequently the short of supply thereof results in increase in price, leading to increase in cost. When gold is used as a protective film, the cost increases further.
A lead frame for semiconductor devices plated on the whole surface with palladium is apt to develop a flash in the step of sealing an IC with mold resin when assembling the IC, and hence an additional step of removing the flash is required. This leads to increase in cost. In a lead frame plated on the whole surface with palladium, further, a large potential difference develops between palladium and a metal of the lead frame substrate, so that it is necessary to interpose nickel or palladium-nickel alloy between palladium and the substrate. At this time, corrosion will occur when nickel or nickel alloy, or iron or iron alloy, is used for the substrate; at the present state, therefore, the above-mentioned method can be applied only to lead frames using copper or copper alloy for the substrate.
As to a solder or a soldering paste which do not use lead and use materials other than palladium, it has been proposed to form a surface layer with a solder or a soldering paste incorporated with such metals as indium, bismuth and zinc in place of lead used in tin-lead solders.
With regard to solder and soldering paste used for reflow soldering, alloys of three-component or four-component system have been proposed which contain, in addition to tin, at least two kinds of metals. As to solder for plating, since it is difficult to control the deposition composition of three-component or four-component alloys in the plating liquor at the time of electroplating, alloys of two-component system comprising tin and other one kind of metal are mostly used. However, alloys comprising tin and additionally indium are hardly used in practice because of high cost of indium. Alloys comprising tin and additionally bismuth can have a low melting point but they are apt to become hard and brittle and hence are poor in processability, so that they can be hardly used for lead frames which require bending in processing. Furthermore, they are poor in solder wettability and hence show a low bond strength and a low heat fatigue strength; moreover, since bismuth tends to deposit at the interface, the so-called lift-off phenomenon, wherein the IC lifts from the solder, is apt to develop at the time of surface mounting. Alloys comprising tin and additionally zinc have melting points near to those of tin-lead solders and moreover zinc is inexpensive; however, since zinc is readily oxidized in the air, when the alloys are subjected to thermal hysteresis in the step of assembling the IC, they tend to be oxidized to cause deterioration of solder wettability.
In recent years, therefore, as to a solder or a soldering paste which do not use lead, alloys comprising tin and additionally silver have been proposed.
More specifically, with regard to a lead frame for semiconductor devices plated with an alloy comprising tin and additionally silver, JP-A-10-102283 discloses xe2x80x9can electric-electronic circuit element having a glossy tin-solver alloy plating film having a content of silver in the film of 0.1%-10% and a film thickness of 0.1-100 xcexcm from a non-cyanide tin-silver alloy electroplating bathxe2x80x9d.
JP-A-2000-164782 discloses xe2x80x9ca semiconductor device having a lead frame formed essentially of nickel or nickel alloy, copper or copper alloy, or iron or iron alloy, wherein the inner lead part is provided with a surface layer of silver or an alloy containing silver and the outer lead part is provided with a surface layer of an alloy at least containing silver and tin of the body-centered tetragonal structure preferentially oriented in the (101) plane and/or the (211) planexe2x80x9d.
The present inventors have made extensive study to provide a lead frame for semiconductor devices which does not contain lead, one of the environmentally harmful substances, exhibits a good solder wettability and a high bond strength and is of a low cost, and resultantly have attained the present invention.
The present invention relates to a lead frame for semiconductor devices which comprises
i. a substrate comprising nickel, copper, iron or an alloy comprising at least one member selected from the group consisting of nickel, copper and iron,
ii. an inner lead part having a surface layer A, the surface layer A comprising silver or an alloy comprising silver, and
iii. an outer lead part having a surface layer B, the surface layer B comprising silver and tin, or copper and tin,
wherein the surface layer B has on its surface an oxidized layer comprising tin and oxygen, the atomic ratio of oxygen to tin in the oxidized layer is 0.5-1.8 and the thickness of the oxidized layer is not more than 20 nm.
The present invention is not bound to any theory. However, it is considered that decreasing the proportion of oxygen by selecting the atomic ratio of oxygen to tin in the range of 0.5-1.8 helps to prevent the formation of stable SnO2 having a high melting point in the oxidized layer and helps to prevent the deterioration of solder wettability. It is further considered that when the thickness of the oxidized layer is selected at not more than 20 nm, the oxidized layer assumes to have an island structure or a structure near to an island structure. Accordingly, the tin of the surface layer B assumes to have a state exposed or nearly exposed to the surface; resultantly diffusion from molten solder and melting at the eutectic temperature are facilitated, leading to further improvement of solder wettability.
The above-mentioned oxidized layer can be formed by swelling a coarse and thick oxidized layer formed on the surface layer B as the result of oxidation with a treating agent preferably containing sodium hydroxide or sodium triphosphate and subjecting the swollen layer to an etching treatment by an electrochemical method.
The silver content of the surface layer B is preferably 1-8% by weight and the copper content is preferably 0.1-2% by weight. The thickness of the layer is preferably 3-15 xcexcm. Such a surface layer B before swelling and etching can be formed by electroplating using as the anode an insoluble electrode containing at least one member selected from the group consisting of platinum, iridium, tantalum, rhodium, ruthenium, and the oxides thereof. For preventing the development of electromigration of silver at the time of IC drive, the tin has preferably the body-centered tetragonal structure.
The process for producing a lead frame for semiconductor devices according to the present invention comprises, for the purpose of forming an oxidized layer comprising tin and oxygen in which the atomic ratio of oxygen to tin is 0.5-1.8 and of which the thickness is not more than 20 nm on the surface of a surface layer B provided to the surface of the outer lead part, a step of swelling the outer lead part having the surface layer B with a treating agent containing sodium hydroxide or sodium triphosphate and a step of subjecting the swollen part to an etching treatment by an electrochemical method.
The present invention further relates to a semiconductor device formed by using the above-mentioned lead frame for semiconductor devices.
According to the present invention, there can be provided a lead frame for semiconductor devices which does not contain lead, one of the environmentally harmful contaminants, exhibits a good solder wettability and high bond strength and is of a low cost, a process for producing the same and a semiconductor device using the lead frame for semiconductor devices.